RNA interference (RNAi), also known as post-transcriptional gene silencing (PTGS), is a phenomenon of shutting down homologous gene expression or gene silencing at mRNA level mediated by double-stranded RNA (dsRNA) molecules, RNAi was first reported in plants in 1990 by two different groups, and later on, this phenomenon was further observed in almost all eukaryotes, including C. elegans, Drosophila, zebrafish and mice (Napoli C, 1990; Fire A, 1991; Guo S, 1995). In 1999, RNA fragments of 21 to 25 nucleotides in length were identified in plant RNAi by Hamilton and Baulcombe. These small RNA fragments were demonstrated to be the mediator necessary for RNAi, and thus named as small interfering RNA (siRNA) (Hamilton A J, 1999). Double-stranded siRNA conjugates with endogenous enzymes and proteins, and then forms RNA-induced silencing complex (RISC). In the process of RNAi, while the sense RNA strand of double-stranded siRNA is released from the complex, the antisense RNA strand functions to guide RISC to target mRNA at homologous locus, resulting in the degradation of target mRNA and gene silencing mediated by RNase III component within the RISC complex (Zamore P D, 2000; Hammond S M, 2001).
In addition to gene function studies, siRNA was extensively used in treatment of human diseases, inhibiting diesease-causing gene expression in major diseases such as viral infection or tumor (Tiemann the K, 2009; Jackson, A L 2010). However, due to the low stability, siRNA is susceptive to degdation by ribonuclease which is abandent in blood (Czauderna F, 2003; Haupenthal J, 2006; Turner J J, 2007), thus synthetic siRNAs are often chemically modified to increase their stability in blood (Braasch D A, 2003; Layzer J M, 2004; Choung S2006). Even though chemical modification can fairly improve stability of siRNA in blood, however, the introduced modification results in increased potential cytotoxicity and compromised its biological activity in many cases. This therefore limits in vivo applications of the modified siRNAs.
Therefore, there is an urgent need to develop a strategy to enhance the stability of synthetic siRNAs in blood without increasing their in vivo toxicity, and hence to solve the technical bottleneck faced in the development of siRNA therapeutics.